An ink jet printing device, which is used as an image forming device in a printer, a facsimile, a copier, a plotter and the like, is provided with an ink jet printhead as a drop discharge head. The ink jet printhead comprises a nozzle for ejecting the ink drops, an ink channel (also referred to as a lip chamber, a pressure chamber, a pressurized drop chamber, or an ink cavity) connected in fluid communication to the nozzle, and a drive mechanism for pressuring ink in the ink channel. Although the following description is mainly related to an ink jet printhead as a drop discharge head, the drop discharge head comprises a head for discharging a liquid resist as a drop and a head for discharging a DNA piece as a drop.
With a piezoelectric ink jet printhead, the volume change of the ink channel resulting from a deformation of a diaphragm using a piezoelectric element causes the ink drops to be expelled (for example, see JP 61-51734A). With another type of ink jet printhead, the bubbles generated by heating ink in the ink channel using a heating resistance element causes the ink drops to be expelled (for example, see JP 61-59911A). With another type of ink jet printhead, the volume change of the ink channel caused by a deformation of a diaphragm as a result of generating an electrostatic force between the electrode and the diaphragm causes the ink drops to be expelled (for example, see JP 61-51734A).
Among these types of ink jet printheads, the piezoelectric ink jet printhead has advantages especially for color printing, because the potential for degradation of the ink drops due to thermal energy is eliminated (especially, the color ink is more likely to be degraded by heat). Furthermore, flexible control of the amount of ink drops can be accomplished by control of the deformation amount of the piezoelectric vibrator. Accordingly, the piezoelectric ink jet printheads are suited for configuring the ink jet printing device with a capability for high quality color printing.
By the way, in order to accomplish a higher quality of color printing, a higher resolution is demanded. To this end, the sizes of the piezoelectric vibrator and the parts related to the ink channel (for example, the partition walls between pressure chambers) are inevitably reduced and thus increased accuracy is required in fabricating and assembling these parts. Under the circumstances, in order to finely fabricate the complicated parts having microstructures such as a pressure chamber, micromachining techniques in which anisotropic etching is applied to a single crystal silicon substrate are proposed. In this case, the parts (for example, a spacer that is arranged between a nozzle plate and a diaphragm and constitutes the pressure chamber) made from single crystal silicon base have higher mechanical stiffness in comparison with the parts made from a photoresist and thus the overall distortion level of the ink jet printhead due to vibration of the piezoelectric vibrator is reduced. Furthermore, it becomes possible to make the pressure chambers uniform, because the etched wall surfaces of the pressure chambers are normal to the surface of the spacer.
JP 7-178908A discloses a printhead made using a micromachining technique, in which the anisotropic etching is applied to a single crystal silicon substrate with crystal orientation (110) to form the pressure chambers. The potion of the pressure chamber adjacent to its outlet is defined by six wall surfaces, that is to say, the four wall surfaces normal to the single crystal silicon substrate, each of which connects to the neighboring wall surfaces at obtuse angles, and two surfaces connected to the particular one of these four wall surfaces at an obtuse angle, from a cross-sectional view of the single crystal silicon substrate. This traditional technique attempts to avoid stagnation of the bubbles by making the ink flow uniform as soon as possible in the area adjacent to the outlet (i.e., the opening on the nozzle plate side) of the pressure chamber where stagnation of the flow is likely to occur.
JP 7-125198A discloses the printhead made using a micromachining technique, in which the potion of the pressure chamber adjacent to its outlet is defined by five wall surfaces normal to the single crystal silicon substrate, each of which connects to the neighboring wall surfaces at an obtuse angle. Further, one wall surface of the pressure chamber is formed by an extended surface of one wall of the reservoir. This traditional technique attempts to eliminate stagnation of the bubbles in the neighborhood of the opening on the nozzle plate side by communicating between the reservoir and the pressure chamber smoothly and locating the outlet of pressure chamber nearly equidistant from the wall surfaces of the pressure chamber.
JP 10-264383A discloses a printhead comprising an ink cavity (pressure chamber) in which ink is pressurized using the piezoelectric element to be expelled outside. A hydrophilic and alkali-proof film, such as nickel oxide and silicon oxide, is deposited on the inner surface of the ink cavity so as to minimize elution of silicon into inks (especially, in the case of using anionic inks).
JP 11-348282A discloses a printhead made by fastening a first substrate to a second substrate having nozzle bores therein using an adhesive. The first substrate has recesses in a staggered arrangement along the edge of the ink cavity and the reservoir. It becomes possible to prevent redundant adhesive from flowing into an ink channel, because the redundant adhesive flows into the recesses.
However, in the case of making the spacer (the component having the ink channel formed therein) from a silicon substrate by etching, it is difficult to process the silicon substrate into a desired structure, because the etching process is dependent on the crystal orientation of the silicon substrate. Furthermore, the etching results in roughness on the silicon surfaces of the pressure chamber.
The aforementioned printheads according to prior art have failed to reduce the stagnation of the bubbles and the retention of ink to a sufficient degree. Especially, having more than four wall surfaces of the pressure chamber results in a detrimental effect on the ink flow due to the multi-dimensional surface structures and makes it difficult to control the ink flow.
Furthermore, in the case of depositing a film of oxide or titanium nitride (fluid (ink) proof film) on the wall surface of the pressure chamber of the spacer for preventing the elution of silicon into inks, the internal stress of the fluid-proof film causes a distortion (bowing) of the overall spacer. If the other components such as the nozzle plate, the diaphragm in the case of the thermal and electrostatic types of printhead, and a cover for constituting the ink channel (for example, a pressure chamber) are fastened to the spacer, it often leads to faulty bonding between these components and the spacer and thus a decrease in reliability.